Stabilized modulated oscillator



, E. R. MANN ETAL' STABILIZED MODULA'IED OSCILLATOR May 25, 1948.

Filed Aug. 5, 1944 H" ks 772mm INVENTbRS f TTOPNEY Patented May 25, 1948STABILIZED MODULATED OSCILLATOR Estle Ray Mann, Upper Montclair, andMorris Spector, Clifton, N. J., assignors to Allen B. Du MontLaboratories, Inc., Passaic, N. J., a

corporation of Delaware Application August 5, 1944, Serial No. 548,158

This invention relates to a modulated oscillator in which provision ismade by means of which the oscillator is kept sensitive and is alsoprevented from becoming overloaded. The OS- cill'ator is therebystabilized.

Heretofore vacuum tube oscillators have been provided for testing orexamining metals by placing pieces of the metals in or passing themthrough an inductance coil that is located in the grid tank circuit ofthe oscillator and modulating the oscillator by means of a low frequencyvoltage, The modulated output from the oscillator may be examined, forexample, by means of an oscillograph; and since the pattern on the faceof the cathode-ray tube of the oscillograph varies with the sort andcondition of the metal placed in or passed through the coil, informationrelating to the metal is thereby rendered available.

Means are also available for selecting-or sorting metal pieces byplacing them in or passing themthrough such coils and utilizing thesignals so produced for sorting or selecting the metal pieces. 7

It is desirable to make such oscillators both stable and sensitive.Stability can usually be se-- cured by using push-pull vacuum tubecircuits in the oscillator and providing symmetry of components and bymodulating the oscillator by means of a stable signal of lowfrequency'compared to that of the oscillator. In this way the oscillatorcan be made to oscillate and still be very stable even very near itsquench point, The low frequency modulator supplies energy at all timesregardless of the amplitude of the high frequency oscillations, therebysustaining the high frequency oscillations, even when they are veryclose to their quench point, so that they do not become entirelyquenched.

However, when the load is removed from the output of the oscillator theamplitude of the high frequency oscillations may increase to such anextent that the oscillator becomes so much overloaded that the outputoscillations become objectionably distorted.

By the present invention, the oscillator is kept sensitive and is alsopreventedfrom becoming overloaded. That is, the oscillator is keptoscillating and the amplitude of the oscillations are prevented frombecoming too large.

The invention may be understood from the description in connection withthe accompanying drawing which is a diagram of connections illustratingthe invention.

In the drawing, reference character I indicates a coil into which themetal to be tested is placed Claims. (Cl. 25036) or through which it ispassed. It is tuned by capaoitor I. A source 2 of low frequency current,say l03-cycles per second, is connected to one end of resistor 3 theother end of this resistor being grounded. A sliding contact 3' onresistor 3 is connected to the center of coil l.

Theends oi;coil l are coupled by leadst and 5 and capacitors 6 and 1 tothe grids of push-pull oscillator tubes 8 and 9 which are provided withgrid leak resistors IE] and II., and with cathode bias resistors l2 andI3 connected to a common variable resistor M which is connectedtoground.

The ends of. coil I are also coupled by leads it and ll and capacitors[8' and IE to the grids of push-pull cathode biased amplifier tubes 20and 2| which are provided with grid leak resistors 22 and 23 that areconnected to intermediate points 24 and 25, on cathode bias resistorsZSand 27 of tubes 20 and 2 '1, respectively.

The platev of tube 8 iscoupled by lead 28, capacitor 29, and variableresistor 30 to ground. An adjustable contact on resistor 30 is connectedby lead 3| to the lead 5. The plate of tube 9 is coupled by lead 32,capacitor 33, and variable resistor 34 to ground. An adjustable contacton resistor 34 is connected by lead 35t0 the lead 4. This provides apush-pull oscillator of the known sort. The plates of tubes 20 and 2|are connected by leads 36 and 31 to a source of positive potential whilethe plates of tubes 8 and 9 are connected through plate load resistors38 and 39 to the samesource of potential.

The cathodes of. tubes 20 and 2! are coupled by capacitors 40 and 4| topotentiometer gain control resistors 42 and 43. The choke coils M and45, which are in parallel with the resistors 42 and 43, are filterstoremove the low frequency modulating componentsthat are introduced bythe alternator 2. The sliding contacts and leads 45 and. 41 couple thegain controls. 42 and it to the grids of tetrodes 48. and49. Thecathodes of these tubes are'biased by resistor 50 that is connected toground as are the adjacent ends of resistors 42, 43- andinductance coils44 and 45.- A resistor 5! is provided in the usual way between a sourceof positive potential for the plates. of tubes-48, 49 and the screengridsof these tubes, and capacitors 52 and 53 are. provided between thescreen grids of these tubes and the: cathodes thereof. and ground. Theload'resistors oftubes 48 and 49 are shown at 54 and 55.

The leads 56and 5'! and coupling capacitors 58 and 59are for applyingthe amplitude modulated high frequency signals on the plates. of tubes48 and 49 to an indicating device not shown,

such as a cathode-ray tube, a meter, a lamp, a trigger circuit or arelay that are well known for observing or utilizing the high frequencyenvelope.

necting them to points 61 and 68, respectively,

on voltage dividers 69, 10 and H, 12 that are connected between sourcesof positive potential coil I one after the other, one of them will moreand ground. As long as the points 61 and 68 are at a potential levelhigher than the ampli tude of a signal applied to the plates of thesediodes 65 and 66 no current will flow through them. The signal fed intothe cathodes of tubes 65 and 66 from the amplifiers 48 and His theamplified amplitude modulated high frequency signal from coil l Thefilter network comprising resistors i3, 14 and 15 and capacitors 16 and82 contains a series resonant circuit consisting of the capacitor 1! andan inductor 18 in series. This resonant circuit 11, I8 is tuned to themodulating frequency which is the frequency of the source 2. A lead 89connects a point between the resistors I0 and ll to one end of thisfilter. A resistor 8| is connected between the plates of diodes 65 and66 and said filter network. I

The operation is as follows:

Apositive potential is applied at the point shown on the drawing abovetube 20, whereupon the push-pull oscillator 8-9 begins to oscillate. Theamplitude of the oscillations is adjusted by varyingthe-resistors I4, 30and 34, the latter two' beingvaried together. This amplitude can beobserved by means of the indicating device mentioned above for,observing the envelopeof these high frequency oscillations. Thealternatori. feeds a low frequency potential of say 103 cycles, persecond tothe coil l.

The amplitude of this potential is adjusted by positioning the contactor3 on resistor 3 to providethedesired' amount of modulating signal. Thismodulating signal aids in keeping the high frequency oscillations stablewithout causing the high frequency oscillator to lose sensitivity.

,The stabilizing effect is also aided by providing symmetry. ofcomponents in the duplicate portions of the oscillator. 7

Due to the core losses in the sample caused by the, oscillatingmagnetic. field of coil I, the amplitude of the high frequencyoscillations is decreased,-thus,causing the peaks of the modulatedenvelope to decrease. Such distortion is obviated with the presentinvention.

The output signals from the plate circuits of tubes 48 and 49 arecoupled by'leads 56 and 51 and condensers58 and 59, respectively,to'diodes and filter circuits which detect the envelope produced by themodulated oscillator. The signals having the amplitude and phase of thisenvelope taken from the leads 56 and 51 may be used in usual waystoproduce useful indications on a cathode ray tube, a meter, or to operatea relay by means of atriggering device, for example.

.If. the amplitude of the oscillations were allowed to increase, withoutautomatic volume control means, the output from the oscillator would bean output from which the envelope had been removed from the amplitudemodulated signal due to, overloading of the oscillator or overloadingthe two amplifiers 48, which follow the oscillator, or overloading both.With this invention, the amplitude of the oscillations are 4 limited toa predetermined maximum so that overloading does not occur and thereforethe envelope is not removed.

The sensitivity of the oscillator is greatest when it is perating nearits quench point as is usual with oscillators of this sort. It istherefore desirable in using the. device for a metal specimen in thetest coil l to bring the amplitude of the oscillations to near zero.When this 'is'done, if two metal specimens differing in-somemetallurgical properties are inserted in the test nearly quench theoscillator than the other. The

amplitude of the oscillations in the one case will be smaller thanitwill be in the other case. The amplitude of the envelope in the onecase will also be smaller than it will be in the other case.

' If, however, the metal specimen is withdrawn from the test coil theamplitude of the oscillations will increase to such an extent thatoverloading will occur and the envelope will be removed unless provisionis made for preventing this.

To prevent this overloading a negative voltage is inserted at the propertime, namely, as the core is being withdrawn, between resistors l0 and Hso that the grids of the oscillator tubes 8 and 9 will be drivensufiiciently negative to keep the amplitude of the oscillations downbelow the overload point. However, it is desirable that this preventionof overloadingzshould not begin when the oscillator is near its quenchpoint. If this were done the voltage fed back by means of the automaticvolume control means would be equivalent to introducing degeneration inthe oscillator and its attendant circuits. The intro-' duction of thisdegeneration would serve the purpose of preventing overloading, but atthe same time would diminish the sensitivity of the instrument sothat itmight not indicate the difference between core losses of two unlikemetal specimens which differ only slightly.

Therefore, some delay is provided for inthe automatic volume controlmeans. This desirable delay is provided by the predetermined voltagethat is applied at points 61 and. 68 taken from the voltage dividers 69,10 and H, 12, respectively, so that the amplitudes of the output signalsfrom the plate circuits of tubes 48 and 49 at-which the diodes 65 and 66are allowed to conduct is predetermined. Signal amplitudes below thevoltages at 61 and 68 do not allow these diodes to conduct. By thismeans the automatic long as the amplitudes of the signals from plates oftubes 48 and 49 are lower than the potentials at 61 and 68, but isapplied when these amplitudes exceed the potentials at 61 and 6B.

By this means all the sensitivity desired for the oscillator is madeavailable without degeneration when it is' operating near its quenchpoint. As the amplitudes increase when a specie men is removed from thetest coil and when sensitivity is no longer desired the automatic volumecontrol introduces degeneration and decreases the sensitivity when highsensitivity is no longer needed. I 1 f The signal rectified by diodesBSand 66 contains a high percentage of the modulating frequency envelope.This rectified signal is filtered by the filter consisting of condensersi6 and 82, and resistors 13, 14 andl5 and the series network consistingof capacitance I1 and inductance 18 which is tuned to the modulatingfrequency. If this modulating "frequency were not removed from therectified signal it would be reinserted at the point between resistorsl0 and II of the oscillator and would remodulate the oscillator. Thisremodulation could be the same or different in both phase and amplitudefrom that inserted by the source 2.

With the delayed automatic volume control circuits, as a metal specimenis inserted in the test coil I, the amplitude of the oscillatordiminishes slowly at first until it drops below a voltage predeterminedby the potentials at the points 61 and 68 because at voltages higherthan this the oscillator is relatively insensitive. When the amplitudeof the oscillations have dropped below this level further insertion ofthe specimen into the coil produces large changes in these amplitudesbecause without this automatic volume control the oscillator would bemuch more sensitive to changes in core losses that are produced by thepresence of the metal that is inserted into the test coil I. With thisinvention the oscillator retains its high sensitivity to smalldiiferences between core losses that are produced by different metalsbeing in the test coil and, at the same time, this oscillator becomesrelatively insensitive to the large core loss difierences which areshown when the test coil has no metal core and when it has a metal coreinserted into it.

What is claimed is:

1. A stabilized oscillator comprising a pair of vacuum tubes each havinga control grid, a source of low frequency current connected to each oneof said control grids to amplitude modulate the oscillations of saidoscillator, push-pull cathode biased amplifiers having control grids towhich the outputs of said pair of vacuum tubes are respectively coupled,a filter for said low frequency current located between the outputterminals of said push-pull amplifiers and ground, a pair of diodeshaving cathodes to which said push-pull amplifiers are coupledrespectively, and a filter for said low frequency current locatedbetween the control grids of said pair of vacuum tubes and the plates ofsaid diodes.

2. The device of claim 1 in which push-pull amplifiers are locatedbetween said filter and said diodes and connected thereto.

3. A stabilized oscillator comprising a pair of vacuum tubes each havinga control grid, a source of low frequency current connected to each oneof said control grids to amplitude modulate the oscillations of saidoscillator, push-pull cathode biased amplifiers having control grids towhich the outputs of said vacuum tubes are respectively coupled, afilter for said low frequency current located between the outputterminals of said push-pull amplifiers and ground, a pair of diodeshaving cathodes to which a source of positive potential and saidpush-pull amplifiers are coupled respectively, and a filter for said lowfrequency current located between the control grids of said oscillatorand the plates of said diodes.

4. The device of claim 1 in which said filter is resonant at thefrequency of said low frequency current.

5. The device of claim 1 in which resistors are located between a sourceof positive potential and the cathodes of said diodes.

ESTLE RAY MANN. MORRIS 'SPECTOR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date,

2,138,138 Bruckner Nov. 29, 1938 2,264,369 Golicke Dec. 2, 19412,343,539 Edson Mar. '7, 1944 2,352,219 Olesen June 27, 1944 FOREIGNPATENTS Number Country Date 426,396 Great Britain Mar. 28, 1935

